702 BELL SYSTEM TECHNICAL JOURNAL 



radiation, or whether the spectral selectivity is in the nature of a locally 

 intrinsic emissive power, such as would be caused by an optical absorp- 

 tion band or an electronic transmission band. In order to answer this 

 question, it is necessary to have emitting surfaces of a specular char- 

 acter. Such surfaces have not been prepared with the alkali hydrides, 

 but it has been found possible to make the caesium-silver-oxide cells 

 on specular plates of silver so that they retain their specular character 

 in the final sensitized surface. Cells of this sort were used in this study, 

 and have made possible a clear separation of the emissive singularities 

 due to optical conditions and the singularities which may be described 

 as intrinsic to the material. 



Both from their method of preparation and from their optical be- 

 havior, we have felt justified in considering the caesium-silver-oxide 

 photoelectric cells prepared with specular silver surfaces as consisting 

 of silver surfaces overlaid with a thick layer of transparent refracting 

 material, on the top of which is a thin photosensitive layer. The 

 silver plates, after oxidation, exhibit interference colors, the exact 

 color depending upon the amount of oxidation. Viewed at an angle 

 through a nicol prism, these oxidized plates exhibit the well-known 

 properties of thin refractive layers on a metal base. Thus when the 

 plane of polarization is changed from the plane of incidence to the 

 plane perpendicular thereto, no change of hue takes place for small 

 angles of incidence; but at large angles, the color changes to a comple- 

 mentary hue. After the silver oxide surface has been exposed to cae- 

 sium vapor and given a heat treatment, these optical properties are 

 still usually observable, but degraded. The softening of the interfer- 

 ence colors may be due either to a change in thickness of the refracting 

 medium as caesium oxide is formed or to the introduction of a general 

 body color. In a few less common cases the colors faded out com- 

 pletely, the plate at the end of the heat treatment being metallic in 

 appearance yet still exhibiting a pronounced selective response to red 

 and infrared light. 



The behavior of a thin photoelectric sheet separated from a specular 

 metal surface by a layer of refracting medium has been treated in an 

 earlier paper where a layer of caesium was deposited on the top of a 

 quartz-coated platinum plate. The data obtained in this earlier paper 

 are immediately applicable to the present problem, granting the similar- 

 ity of conditions which we have assumed. It has been convenient to 

 pursue this present study on the assumption of such a similarity and to 

 arrive at conclusions from the agreement with, or deviation from, the 

 results obtained from the simpler materials and conditions previously 

 studied. 



